1. Field of the Invention
The present invention relates to an apparatus for moving a curved-surface minor.
2. Description of the Related Art
An exposure apparatus is used in a photolithographic process for the manufacture of a semiconductor element, to expose the circuit pattern of a reticle, and to transfer the pattern to a silicon wafer, by way of example. Owing to a demand for higher integration of semiconductor elements, step-and-scan exposure systems now dominate and have supplanted the conventional step-and-repeat exposure systems. A step-and-scan exposure apparatus is a scanning exposure apparatus in which slit-shaped illuminating light is partially emitted from one end of a reticle, and the entirety of a circuit pattern is exposed on a wafer (substrate) via a projection optical system, while a reticle stage holding the reticle and a wafer stage holding the wafer, are driven synchronously.
In an exposure apparatus of this kind, the resolution of the projection optical system has been improved in order to expose and to transfer a fine pattern accurately. For example, the apparatus naturally is designed in order to enlarge the numerical aperture (NA) of the optical system and to reduce optical aberration, and manufacture, assembly and adjustment, employing highly sophisticated measurements, have been carried out. Furthermore, it is desired that various optical characteristics (e.g., magnification, distortion and focus) be adjustable during the exposure operation. In addition, it is necessary that the effects of disturbance (e.g., fluctuations in air pressure, fluctuations in temperature and humidity, floor vibration and aging) be eliminated, to a maximum degree, in order to achieve stable performance. In order to realize these functions, the positions, attitudes and deformations of the optical elements (e.g., lenses, mirror and prisms) that construct the projection optical system should be adjusted during the exposure operation.
A variety of types of optical element driving apparatuses have been developed from this standpoint. For example, the specification of Japanese Patent Laid-Open No. 2002-131605 discloses an optical element driving apparatus, in which the optical elements are capable of being driven through six-degrees-of-freedom of motion.
Further, the specification of Japanese Patent Laid-Open No. 2004-064076 discloses an optical element driving apparatus, in which a number of focus actuators are deployed on the underside of a mirror, to adjust the shape of the reflective surface of the mirror, and position sensors are provided to enable adjustment of rotation about the direction of the optical axis and two other directions that form an orthogonal system with the direction of the optical axis.
In general, if the number of drivable optical elements or number of degrees of freedom of motion is increased, the optical characteristics of a projection optical system can be adjusted to more preferable characteristics. However, unless other components and displacement errors can be sufficiently suppressed, with respect to drive in a desired direction and drive displacement, optical characteristics will worsen instead. This manifests itself, in particular, when adjusting and driving a mirror, the optical sensitivity of which is greater than that of a lens. Further, if the rigidity of the mechanism declines, owing to an increase in degrees of freedom of drive, positional deviation may increase, owing to disturbances, even if it is possible to adjust the drive to the desired position, and optical characteristics may worsen. For these reasons, it is required that driving of the mirror be executed by a driving apparatus having highly sophisticated mechanical and control systems.